Method of etching microelectronic mechanical system features in a silicon wafer

ABSTRACT

A method of etching features in a silicon wafer includes coating a top surface and a bottom surface of the silicon wafer with a mask layer having a lower etch rate than an etch rate of the silicon wafer, removing one or more portions of the mask layer to form a mask pattern in the mask layer on the top surface and the bottom surface of the silicon wafer, etching one or more top surface features into the top surface of the silicon wafer through the mask pattern to a depth plane located between the top surface and the bottom surface of the silicon wafer at a depth from the top surface, coating the top surface and the one or more top surface features with a metallic coating, and etching one or more bottom surface features into the bottom surface of the silicon wafer through the mask pattern to the target depth plane.

TECHNICAL FIELD

The present specification generally relates to methods of etchingfeatures into silicon substrates and, more specifically, to methods ofetching silicon substrates using metal layers and/or a specific order ofsteps.

BACKGROUND

Microelectronic mechanical systems (“MEMS”) may be formed by etchingfeatures into one or more silicon wafers. Features may be etched in asilicon wafer using a number of techniques. One such technique isanisotropic deep silicon etching. For example, micro- or nanopillararrays, accelerometers, complementary metal-oxide semiconductors, andmicro- or nanofluidic devices may have one or more features that havebeen formed using anisotropic deep silicon etching.

SUMMARY

In one embodiment, a method of etching features in a silicon waferincludes coating a top surface and a bottom surface of the silicon waferwith a mask layer having a lower etch rate than an etch rate of thesilicon wafer, removing one or more portions of the mask layer to form amask pattern in the mask layer on the top surface and the bottom surfaceof the silicon wafer, etching one or more top surface features into thetop surface of the silicon wafer through the mask pattern to a depthplane located between the top surface and the bottom surface of thesilicon wafer at a depth from the top surface, coating the top surfaceand the one or more top surface features with a metallic coating, andetching one or more bottom surface features into the bottom surface ofthe silicon wafer through the mask pattern to the target depth plane.

In another embodiment, a method of etching one or more features in asilicon wafer includes coating a top surface and a bottom surface of thesilicon wafer with a mask layer having a lower etch rate than an etchrate of the silicon wafer, removing one or more portions of the masklayer to form a mask pattern in the mask layer on the top surface andthe bottom surface of the silicon wafer, etching one or more bottomsurface features into the bottom surface of the silicon wafer throughthe mask pattern to a target depth plane located between the top surfaceand the bottom surface of the silicon wafer at a target depth from thetop surface, coating the bottom surface and the one or more bottomsurface features etched into the bottom surface with a metallic coating,and etching one or more top surface features into the top surface of thesilicon wafer through the mask pattern to the target depth plane.

In yet another embodiment, a method of etching one or morethrough-features into a silicon wafer including a top surface and abottom surface and coated in a mask layer is described. The one or morethrough-features include one or more nozzle through-holes, an outletplenum, and a cooling fluid outlet. The method includes forming a maskpattern in a mask layer top surface and a mask layer bottom surface ofthe mask layer, etching a bottom portion of the one or more nozzlethrough-holes from the bottom surface to a nozzle target depth throughthe mask layer bottom surface, etching the cooling fluid outlet from thebottom surface to the plenum target depth through the mask layer topsurface, coating the bottom surface and the bottom portion of the one ormore nozzle through-holes and the cooling fluid outlet with a metalliccoating, etching a top portion of the one or more nozzle through-holesfrom the top surface to the nozzle target depth, and etching the outletplenum from the top surface to a plenum target depth. The first of theone or more through-features to etch through the silicon wafer iscompleted at an initial through-etch time, and the last of the one ormore through-features to etch through the silicon wafer is completed atan etch completion time.

These and additional features provided by the embodiments describedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1A schematically depicts an example embodiment of an embeddedmicro-channel cooling system including a manifold substrate and acooling substrate, according to one or more embodiments shown anddescribed herein;

FIG. 1B schematically depicts a cross-sectional view of the embeddedmicro-channel cooling system of FIG. 1A along the line 1B-1B of FIG. 1A,according to one or more embodiments shown and described herein;

FIG. 2 schematically depicts an example process for etching the exampleembodiment of the embedded micro-channel cooling system of FIG. 1A,according to one or more embodiments shown and described herein;

FIG. 3 depicts a flow diagram of the example process of FIG. 2,according to one or more embodiments shown and described herein;

FIG. 4A schematically depicts an exploded view of an example embodimentof a multi-substrate layer cooling device including a nozzle array and acooling channel array, according to one or more embodiments shown anddescribed herein;

FIG. 4B schematically depicts a detailed view of the nozzle array of themulti-substrate layer cooling device of FIG. 4A, according to one ormore embodiments shown and described herein;

FIG. 4C schematically depicts a detailed view of the cooling channelarray of the multi-substrate layer cooling device of FIG. 4A, accordingto one or more embodiments shown and described herein;

FIG. 5 schematically depicts a top surface of the second substrate layerof the multi-substrate layer cooling device of FIG. 4A, according to oneor more embodiments shown and described herein;

FIG. 6 schematically depicts a bottom surface of the second substratelayer of the multi-substrate layer cooling device of FIG. 4A, accordingto one or more embodiments shown and described herein;

FIG. 7 schematically depicts an example process for etching the exampleembodiment of the multi-substrate layer cooling device of FIG. 4A,according to one or more embodiments shown and described herein;

FIG. 8 depicts a flow diagram of the example process of FIG. 7,according to one or more embodiments shown and described herein; and

FIG. 9 depicts a flow diagram of another example process for etching themulti-substrate layer cooling device of FIG. 4A, according to one ormore embodiments shown and described herein.

DETAILED DESCRIPTION

Embodiments described herein relate to methods of forming a chip-scalecooling device from a silicon wafer by using an anisotropic etchingmethod on both sides of the silicon wafer. The chip-scale cooling devicemay be integrated with a power semiconductor device. Power semiconductordevices, such as diodes, switches, and amplifiers generate large amountsof heat during operation. Power semiconductor devices, particularly SiCand GaN devices, may switch relatively high levels of current on and offat a high speeds and generate relatively high levels of heat due tooperational losses. Accordingly, large amounts of heat may be removedfrom the devices during operation. Such large amounts of heat may beremoved by integrating one or more chip-scale cooling devices with thepower semiconductor device.

Further, heat reduction of the power semiconductor device may beaugmented using one or more heat removal features formed in the coolingdevice, such as arrays of cooling channels and cooling fluid flow toconduct heat away from the power semiconductor device or other heatgenerating device. The chip-scale cooling device may include featuressuch as a cooling fluid flow path, one or more nozzle structures, one ormore through-silicon vias (TSVs), and one or more cooling channelarrays. The one or more nozzles may impinge fluid onto the array ofcooling channels. The cooling fluid may even change phase as it passesthrough the nozzle to remove an even greater quantity of heat from theheat generating device. It is noted that the concepts described hereinmay be used to cool any type of semiconductor device and are not limitedto power semiconductor devices.

The methods described herein involve etching features of the chip-scalecooling device into opposite surfaces of the device, one surface at atime, to ensure particular shape and/or thickness characteristics, andto preserve the structural integrity of the device. The order that theopposite surfaces are etched may be chosen based on a number ofcriteria. Additionally, one or more layers may be added to afirst-etched surface before the second-etched surface is etched. Thismay neutralize unwanted etching in the first-etched surface. Suchfeatures and processes will be described in greater detail herein.

Anisotropic etching is etching in which the etch rate is different inone dimension (e.g., a lateral etch rate) than in another dimension(e.g., a through-wafer etch rate). Features with high aspect ratios maybe achieved using anisotropic deep silicon etching techniques because itmay be possible to etch through the wafer at a higher rate than alongthe surface of the wafer. However, etching from a single side of a wafermay have particular challenges that require various techniques toovercome.

One useful anisotropic silicon etching technique is a time-multiplexedalternating process (also known as a “Bosch process”). This techniquemay be used for etching relatively deep and relatively narrow featuresthrough thick silicon substrates. However, current time-multiplexedalternating processes may have certain limitations.

For example, in substrates having a pattern of individually etchedfeatures with a small pitch between each feature, such as a pattern ofTSVs, the wall between each feature may be thin, which may lead tostructural instability. If the depth of the feature to be etched isgreater than the thickness of the wall between each of the individualfeatures, the lateral etch rate may be tightly controlled with respectto the through-wafer etch rate to ensure that the dimensions of eachfeature do not interfere with one another and result in one large,overlapping, indistinct feature. Complicating the problem, in featureswith high aspect ratios, the lateral etch rate may increase withincreasing etch depth within the wafer. This may result in lowering wallthicknesses and structural instability between etched features near thebottom of the etched features (i.e., at the side opposite where the etchbegan).

Additionally, to etch features deeply into silicon substrates, thesilicon substrates may be exposed to chemical etchants for long periodsof time. As a result, portions of the substrate that will not be etchedmay require thick mask layers, which will slowly erode as the substrateis etched. Hence, thick silicon-oxide mask layers may be applied to thesubstrate before the etching can begin. The thicker the mask layer, themore difficult it may be to apply and subsequently remove selectportions of the silicon-oxide mask layer to etch the design features.

Further, for features etched in a substrate with different aspect ratios(e.g., a wide inlet plenum and a TSV), it may be difficult to balancethe etch rates of each of the features to ensure that the features arenot over etched, thus exceeding or otherwise not meeting engineeredspecifications. Moreover, if a particular feature is completely etchedbefore another connecting feature, the gases used to etch the featuresmay bleed through the first-etched feature into the second-etchedfeature, thereby interfering with the etch of the second-etched feature(e.g., increasing the lateral etch rate of the second-etched feature).

Even more, in etching methods that utilize two-sided etching to etch oneor more holes through the entire substrate, gases may diffuse throughthe etched hole once the hole passes completely through the substrate(i.e., both sides are completely etched through). If the substrate ismounted to a supporting wafer using mounting oil, the gas used to etchthe substrate may diffuse through the hole formed before completion ofone or more of the remaining features. The gas may then interact withthe mounting oil, causing a waste product to be generated. This wasteproduct may be hard to remove using methods such as wafer cleaning orplasma ashing. Therefore, it may be advantageous to etch from both sidesof a wafer when forming a semiconductor device from a silicon wafer(e.g., a power semiconductor device). Additionally, the side of thesilicon wafer from which the etching process begins may be selectedbased on one or more aspects of the features on the wafer to be etchedand/or based on aspects of the wafer itself.

For example, the number of individual features to be etched into the topsurface or into the bottom surface may be a factor in whether to etchfrom the top surface or the bottom surface first. Additionally, theaspect ratio of the features to be etched into the top surface and thebottom surface may be a factor in whether to etch from the top surfaceor the bottom surface first.

The features of the chip-scale cooling device may be etched into asilicon substrate or wafer using an anisotropic deep etching technique.Various problems may be associated with anisotropic deep silicon etchingthat may be solved using the various techniques described herein. Forexample, all through-wafer features may be etched by starting theetching at both sides of the substrate thereby reducing the etch depthrequired for a given through-wafer feature on either side of thesubstrate and reducing the required thickness of the mask layer.Additionally, because etched features need only extend a fraction ofwhat they would need to be etched if they were etched through a singleside of the substrate, it may be easier to balance the vertical etchrate and the lateral etch rate through the etch. Thus, features etchedthrough the thickness of the substrate are subject to less lateral etchcreep. Moreover, because the thickness of the mask layer is reduced, itmay be easier to form openings in the mask layer, reducing the overalldifficulty of forming openings to etch subsequent features in the masklayer. Each of these developments may reduce the time and resource costof etching new silicon chips, thereby decreasing the cost of productionfor an individual chip and increasing production efficiency.

Embodiments described herein include reducing unwanted etching in asilicon wafer by coating surfaces of the wafer that have been etchedwith a passivation layer (e.g., sputtered aluminum), before etching froman opposite side of the wafer. Coated surfaces will not react with achemical etchant that may inadvertently contact the coated surface.Therefore, the dimensions of the coated surfaces can be more tightlycontrolled, resulting in greater precision in etched features and betterfunctionality of the systems they form.

FIG. 1A shows an example embodiment of an embedded micro-channel coolingsystem 100 including a manifold substrate 102 and a cooling substrate104. In embodiments, the manifold substrate 102 and the coolingsubstrate 104 may be aligned and directly bonded to one another, asdepicted in FIG. 1A. The embedded micro-channel cooling system 100 maybe used to cool one or more semiconductor devices, such as, for example,one or more power semiconductor devices.

Non-limiting examples of power semiconductor devices that may be cooledby the embedded micro-channel cooling system 100 described hereininclude, but are not limited to, SiC semiconductors, GaN semiconductors,or other types of semiconductor devices that provide large bandgaps,high breakdown voltages, and high thermal conductivity. Powersemiconductor devices may be capable of greater capacity in particularaspects than other semiconductor devices, such as higher blockingvoltages, higher switching frequencies, and higher junctiontemperatures. Consequently, they may also require greater coolingcapacity. Implementations of power semiconductors may include, but arenot limited to, bipolar junction transistors (BJTs), insulated-gatebipolar transistors (IGBTs), and power metal-oxide-semiconductorfield-effect transistors (MOSFETs). Power semiconductors may be used aspower supplies, for example, as the power supply for an electricvehicle.

Components of the embedded micro-channel cooling system 100 may beetched from one or more silicon-oxide or silicon wafers. Brieflyreferring to FIG. 2, the wafers used to create the one or more featuresand components of the embedded micro-channel cooling system 100 maycomprise a silicon wafer 136 surrounded by a silicon-oxide mask layer138. The components and features of the embedded micro-channel coolingsystem 100 may be etched from the wafers using one or more etchingprocesses, as described herein.

Referring to FIGS. 1A and 1B, the manifold substrate 102 may include abottom surface 106 that includes one or more inlet holes 108 and a topsurface 110 that includes one or more inlet manifolds 112. The inletmanifolds 112 may be a three-dimensional void or space for receivingcooling fluid in the manifold substrate 102 before the cooling fluid isused in an embedded micro-channel cooling array 122 of the coolingsubstrate 104 to cool one or more power semiconductor devices that maybe coupled to the cooling substrate 104 at a cooling location 140. Theinlet holes 108 and the inlet manifolds 112 may be an inlet portion of acooling fluid flow path 10 represented by arrows 12 in FIG. 1B. As shownin FIG. 1A, the inlet manifolds 112, or portions thereof, may be roundedto minimize disturbance or turbulence in the cooling fluid flow.

Still referring to FIGS. 1A and 1B, the inlet holes 108 may be etchedinto the bottom surface 106 of the manifold substrate 102. Innon-limiting example embodiments, the inlet holes 108 may be circular orsemi-circular in shape. The inlet holes 108 may extend from the bottomsurface 106 to the inlet manifolds 112, thereby fluidly coupling anexternal system that contains cooling fluid to the inlet manifolds 112through the inlet holes 108, and ultimately fluidly coupling theexternal system to an embedded micro-channel cooling array 122 in thecooling substrate 104.

The inlet manifolds 112 may be etched in the top surface 110 of themanifold substrate 102. In the non-limiting example embodiment shown inFIG. 1A, the inlet manifolds 112 are rounded etched portions of themanifold substrate 102. The inlet manifolds 112 may be etched into themanifold substrate 102 using an etching procedure such as the exampleetching procedure described herein. As shown in FIG. 1B, the top of theinlet manifold 112 may be sealed by a bottom surface 116 of the coolingsubstrate 104. Thus, the bottom surface 106 of the cooling substrate 104may prevent cooling fluid from escaping from the inlet manifold 112,which together with the inlet holes 108 form a void that passes throughthe entire thickness of the manifold substrate 102. The bottom surface116 thus forms a portion of the cooling fluid flow path 10 and ensuresthat cooling fluid flows through the cooling fluid flow path 10, asdepicted in FIG. 1B.

Referring to FIG. 1A, the manifold substrate 102 may also include one ormore auxiliary channels 114 integrated therein. Each of the auxiliarychannels 114 may be a channel that receives one or more testingapparatuses that are used to test one or more aspects of the coolingfluid flowing through the cooling fluid flow path 10. For example, theauxiliary channels 114 may enable testing of a pressure and a velocityof the cooling fluid in the cooling fluid flow path 10 (shown in FIG.1B) using a differential pressure detector. The auxiliary channels 114may have a generally tapered profile along their length dimension L. Theprofile may taper along the length dimension L toward the cooling fluidinlet channel opening 120.

The inlet manifolds 112 may be fluidly coupled to the one or morecooling fluid inlet channels 118. The cooling fluid inlet channelopenings may have a relatively wide inlet profile and a relativelynarrow exit profile through the cooling fluid inlet channel opening 120(e.g., a triangular-shaped profile). This profile may increase flowvelocity through the cooling fluid inlet channel opening 120, maydecrease the pressure of the cooling fluid that may flow through thecooling fluid inlet channel opening 120, or both.

The cooling fluid inlet channels 118 may fluidly couple the inletmanifolds 112 with an embedded micro-channel cooling array 122 in thecooling substrate 104. The embedded micro-channel cooling array 122 mayinclude embedded micro-channel cooling array inlets and embeddedmicro-channel cooling array outlets (i.e., inlet and outlet holes) on afluid-coupling side 127 of the embedded micro-channel cooling array 122.The embedded micro-channel cooling array inlets and embeddedmicro-channel cooling array outlets may fluidly couple one or moreembedded micro-channel cooling array cooling channels 124 with thefeatures in the manifold substrate 102 that comprise the cooling fluidflow path 10 shown in FIG. 1B. The embedded micro-channel cooling array122 may be thermally coupled to one or more power semiconductor devicesat the cooling location 140 above the embedded micro-channel coolingarray 122 shown in FIG. 1B. The embedded micro-channel cooling array 122may remove heat generated by the one or more power semiconductor devicesand transfer it to the cooling fluid. The embedded micro-channel coolingarray outlets may be fluidly coupled with one or more cooling fluidoutlet channels 126 in the manifold substrate 102 such that the embeddedmicro-channel cooling array outlets create a path for cooling fluid toflow from the embedded micro-channel cooling array cooling channels 124in the embedded micro-channel cooling array 122 to the one or morecooling fluid outlet channels 126 in the manifold substrate 102.

The cooling fluid outlet channels 126 in the manifold substrate 102 maybe one or more rectangular etched voids etched from the manifoldsubstrate 102 to create an outlet path for the cooling fluid after thecooling fluid flows through the embedded micro-channel cooling array122. As shown in FIG. 1A, some of the cooling fluid outlet channels 126may be located between cooling fluid inlet channels 118. The non-etchedportion of the manifold substrate 102 between the cooling fluid inletchannels 118 and the cooling fluid outlet channels 126 may include amanifold substrate wall 128. The manifold substrate wall 128 may createa barrier that prevents cooling fluid from flowing directly from thecooling fluid inlet channels 118 to the cooling fluid outlet channels126 and thereby bypassing the embedded micro-channel cooling array 122.Said another way, the manifold substrate wall 128 ensures that coolingfluid flows to the embedded micro-channel cooling array 122 bypreventing cooling fluid from entering the cooling fluid outlet channels126 until it has passed through the embedded micro-channel cooling array122. The thickness of the manifold substrate wall 128 may be affected byaspects of the etching process, such as vertical and lateral etchingrates, as described in greater detail herein.

The cooling fluid outlet channels 126 may be fluidly coupled to one ormore external cooling fluid systems. The cooling fluid may exit themanifold substrate 102 through the cooling fluid outlet channels 126 andflow to the one or more external cooling fluid systems. As anon-limiting example, the cooling fluid may exit the cooling fluidoutlet channels 126 to one or more external heat exchangers, such as aradiator, to one or more cooling fluid reservoirs, to one or morecooling fluid recycling systems, or to one or more other cooling fluidsystems.

Referring to FIG. 1B, the cooling fluid flow path 10 will be described.The cooling fluid flowing through the manifold substrate 102 flows alongthe cooling fluid flow path 10. The cooling fluid flow path 10 startswith cooling fluid flowing from an external system, such as a radiatorsystem or a cooling fluid reservoir, for example. The cooling fluidflows into the manifold substrate 102 through the cooling fluid inletholes 108. The cooling fluid that flows into the manifold substrate 102will be relatively cold, because it has not yet absorbed heat from thepower semiconductor device or other heat generating device coupled tothe cooling substrate 104. As shown in FIG. 1B, the manifold substrate102 may include two inlet holes 108, but embodiments that comprise feweror more than two inlet holes 108 are contemplated. The cooling fluid mayflow upward from the inlet holes 108 to the inlet manifolds 112. Fromthe inlet manifolds 112, the cooling fluid may flow inward toward thecooling fluid inlet channels 118.

In the cooling fluid inlet channels 118, the cooling fluid flows upwardto inlet holes in the micro-channel cooling array 122 (not shown,because they are on a downward-facing side of the micro-channel coolingarray 122). The cooling fluid then flows in the micro channels of themicro-channel cooling array, where it absorbs heat from the powersemiconductor device or other heat generating device before it flows outof the micro channels through the outlet holes in the micro-channelcooling array 122 (not shown, because they are on a downward facing sideof the micro-channel cooling array 122). From the micro-channel coolingarray 122, the cooling fluid flows out of the manifold substrate 102through the cooling fluid outlet channels 126 to an external system,such as a radiator system or a cooling fluid reservoir.

The cooling fluid that flows through the embedded micro-channel coolingsystem 100 may include, as one example, deionized water. Other exemplaryfluids include, without limitation, water, organic solvents, andinorganic solvents. Examples of such solvents may include commercialrefrigerants such as R-134a, R717, and R744. Moreover, in someembodiments, the cooling fluid may be a dielectric cooling fluid.Non-limiting dielectric cooling fluids other than deionized waterinclude R-245fa and HFE-7100. The type of cooling fluid chosen maydepend on the operating temperature of the one or more powersemiconductor devices to be cooled. Further, selection of thecomposition of the cooling fluid may be based on, among otherproperties, the boiling point, the density, and/or the viscosity of thecooling fluid.

The manifold substrate 102 may be bonded to the cooling substrate 104.As a non-limiting example, the manifold substrate 102 and the coolingsubstrate 104 may be directly bonded. As used herein, the term “directlybonded” or a “direct bond” (also referred to as “silicon direct bond” or“silicon fusion bond”) means a bond between layers of silicon substrate,such as the manifold substrate 102 and the cooling substrate 104,without an additional layer between the two layers. The manifoldsubstrate 102 and the cooling substrate 104 may be bonded to create thecooling fluid flow path described herein.

In some embodiments, before the manifold substrate 102 and the coolingsubstrate 104 are bonded, they may be aligned. To align the manifoldsubstrate 102 and the cooling substrate 104, a vision-assist aligningprocedure using a machine vision system and/or machine vision may beused. The machine vision system may include one or more optical orinfrared cameras designed to detect one or more fiducial marks 127, asshown in FIG. 2, on the substrate layers and/or one or more visual orinfrared light sources to illuminate the one or more fiducial marks 127in visual or infrared light. The visual or infrared light source mayilluminate the one or more fiducial marks to increase the contrast ofthe fiducial mark 127 from the substrate layer or other feature wherethe fiducial mark 127 is located. The fiducial mark 127 or marks maycomprise one or more opaque or other markings on a surface or otherfeature of a substrate layer and a real-time image capture of thefiducial mark 127 may be compared to a reference image to align thesubstrate layer or layers and the features thereon.

Still referring to FIG. 2, one example process for etching features,such as, for example, the features described herein, into the siliconwafer 136 is shown. FIG. 3 depicts a flow diagram of the example processshown in FIG. 2. The example process described herein may be used, forexample, to create the manifold substrate 102 described herein.Accordingly, Steps 1-6 in FIG. 2 show an example process for making themanifold substrate 102, but it is to be understood that the principlesand explicit steps disclosed herein could be used to etch other featuresinto one or more silicon wafers. Examples of etching processes mayinclude chemical etching processes using a liquid or gas etchant.

Referring to both FIGS. 2 and 3, before the silicon wafer 136 is etched,the silicon wafer 136 may be coated with a mask layer at block 305 andshown at Step 1. For example, the silicon wafer may be coated with asilicon-oxide mask layer 138 such that portions of the silicon wafer 136that need not be etched during a particular step are protected by thesilicon-oxide mask layer 138 and are not etched. In some embodiments,the silicon-oxide mask layer 138 may coat substantially all surfaces ofthe silicon wafer 136. As shown in FIG. 2, step 1, the silicon-oxidemask layer 138 may comprise a mask layer top surface 110′ and a masklayer bottom surface 106′. The mask layer top surface 110′ and the masklayer bottom surface 106′ may cover the top surface 110 and bottomsurface 106 of the silicon wafer 136, respectively.

At block 310 and as shown at Step 2, one or more portions of thesilicon-oxide mask layer 138 may be removed, such as one or more masklayer top surface features 144 and one or more mask layer bottom surfacefeatures 145 forming aa mask pattern 139 (i.e., cutout portions of thesilicon-oxide mask layer 138 that correspond to the featuresto-be-etched into the silicon wafer 136), such that particular features(e.g., the cooling fluid inlet channels 118) can be patterned in thesilicon wafer 136. In some embodiments, portions of the silicon-oxidemask layer 138 may be removed from both the top surface 110 and thebottom surface 106 of the silicon wafer 136, exposing portions of thesilicon wafer 136 located beneath the silicon-oxide mask layer 138. Themask layer top surface features 144 and the mask layer bottom surfacefeatures 145 may correspond to the features that will become thefeatures of the manifold substrate 102, such as, for example, the inletholes 108, inlet manifolds 112, auxiliary channels 114, cooling fluidinlet channels 118, and the cooling fluid outlet channels 126 (FIGS. 1Aand 1B). The silicon wafer 136 and silicon-oxide mask layer 138 shown inStep 1 of FIG. 2 is cut along a midpoint line A-A of the silicon wafer136 and silicon-oxide mask layer 138 to show the example process frominside the silicon wafer 136 and silicon-oxide mask layer 138. Somefeatures, such as the cooling fluid outlet channels 126 and the coolingfluid inlet channels 118 are shown as dashed lines indicating the extentof the features into the thickness of the silicon wafer 136.

In the particular example embodiment shown in FIG. 2, the cooling fluidinlet channels 118 do not include cooling fluid inlet channel openings120 that are wider near the inlet manifolds 112, however, it is to beunderstood that other example embodiments may include this feature.

Still referring to FIGS. 2 and 3, once the portions of the silicon-oxidemask layer 138 are removed and the portions of the silicon wafer 136that will form the features of the manifold substrate 102 are exposed,the silicon wafer 136 may be etched at block 315. The etching processmay be completed as a plurality of steps. For example, the top surface110 of the silicon wafer 136 may be etched first and the bottom surface106 of the silicon wafer 136 may etched second (i.e., subsequent toetching the top surface 110). As shown at Step 3, the top surface 110 ofthe silicon wafer 136 may be etched.

The features that are etched into the top surface 110 of the manifoldsubstrate 102 may include, but are not limited to, the inlet manifolds112, the auxiliary channels 114, the cooling fluid inlet channels 118,and the cooling fluid outlet channels 126. Because certain features thatextend through the entire thickness of the silicon wafer 136 (e.g., thecooling fluid outlet channels 126), such features may be etched in oneor more etching steps. As such, only portions of the inlet manifolds112, the auxiliary channels 114, the cooling fluid inlet channels 118,and the cooling fluid outlet channels 126 may be etched in the topsurface 110 of the manifold substrate 102, as shown at Step 3. Forexample, a top portion 130 of the cooling fluid outlet channels 126 maybe etched into the top surface 110 of the manifold substrate 102 and abottom portion 132 (shown at Step 6 of FIG. 2) of the cooling fluidoutlet channel 126 may be etched in the manifold substrate 102 during adifferent step in the example process, as described herein.

The features etched into the top surface 110 of the silicon wafer 136may be etched to a target depth plane 135 that is formed between the topsurface 110 and the bottom surface 106 at a target depth 134 from thetop surface 110 of the silicon wafer 136. The target depth 134 may be afraction of the overall thickness of the silicon wafer 136. In onenon-limiting example, the target depth 134 may be between about 600micrometers and about 800 micrometers. In other embodiments, the targetdepth 134 may be between about 650 micrometers and about 750micrometers. In other embodiments, the target depth 134 may be betweenabout 675 micrometers and about 725 micrometers. In one non-limitingexample, the target depth may be about 700 micrometers.

Once the features are etched into the top surface 110 of the siliconwafer 136, one or more of the features may be coated to protect thefeatures already etched into the top surface 110 during the etching ofthe bottom surface 106 at block 320. As one non-limiting example of thecoating process, the etched features may be coated with a metalliccoating, such as, for example, an aluminum coating 142 using an aluminumsputtering process. As shown at Step 4, the aluminum coating 142 coatsthe exposed surfaces of the inlet manifolds 112, the top portion 130 ofthe cooling fluid outlet channels 126, and the cooling fluid inletchannels 118. It should be understood that surfaces of the manifoldsubstrate 102 coated during the coating process may use a material otherthan aluminum as a coating. For example, the surfaces coated during thecoating process may be coated using gold or silver.

After the features have been etched and coated, the silicon wafer 136may be flipped so that one or more features may be etched into thebottom surface 106 at block 325. In some embodiments, the silicon wafer136 may be mounted on a carrier substrate while the bottom surface 106is etched. In the particular example embodiment shown in FIG. 2, theinlet holes 108 and the bottom portion 132 of the cooling fluid outletchannels 126 are etched into the silicon wafer 136, as shown at Step 5.Still referring to FIGS. 2 and 3, the bottom portion 132 of the coolingfluid outlet channels 126 may be etched until the bottom portion 132meets the top portion 130 at the target depth plane 135, resulting incertain portions of the silicon wafer 136 (e.g. locations containing thecooling fluid outlet channels 126) passing through the entire thicknessof the silicon wafer 136. Similarly, the inlet holes 108 may be etchedinto the bottom surface 106 of the silicon wafer 136 such that theyinterface the inlet manifolds 112, thereby forming a fluid inlet pathwaythrough the entire thickness of the silicon wafer 136.

It should be understood that the aluminum coating 142 may remain on theexposed features of the top surface 110 during the etching of the bottomsurface 106. During the etching of the bottom surface 106, etch gases orother chemical etchant may be introduced to etch one or more featuresinto the bottom surface 106. The aluminum coating 142 may prevent theetch gas from affecting the features etched into the top surface 110.For example, the aluminum coating may act as a physical barrier (e.g., aplug), thereby preventing gas from passing through holes in the siliconwafer 136 that begin to develop as the etch gas passes through theentire thickness of the silicon wafer 136. Additionally, even if somechemical etchant is able to pass from the bottom surface 106 of thesilicon wafer 136 to the top through holes etched through the thicknessof the silicon wafer 136, the aluminum coating 142 may prevent thechemical etchant from reacting with the portions of the silicon wafer136 that are covered by the aluminum coating 142.

Additionally, the chemical reactions necessary to etch the bottomsurface 106 may be exothermic (i.e., generate heat within the siliconwafer 136). This heat may be concentrated in areas of the silicon wafer136 where etching is occurring, for example, at the portions of thesilicon wafer 136 that will become the inlet holes 108 and the bottomportions 132 of the cooling fluid outlet channels 126. Heatconcentrations may lead to defects in the silicon wafer 136. Forexample, heat may cause portions of the silicon wafer 136 to expandrapidly, melt, or develop gas bubbles at the interface between thesilicon wafer 136 and the aluminum coating 142 or the interface betweenthe silicon wafer 136 and the silicon-oxide mask layer 138. Accordingly,the aluminum coating 142 may act as a heat distributing apparatus thatdistributes heat across the silicon wafer 136 during etching of thesilicon wafer to avoid defect formation in the silicon wafer 136.

Because the aluminum coating 142 is in direct contact with the exposedfeatures in the top surface 110 of the silicon wafer 136, heat generatedin the silicon wafer 136 due to the etching of the bottom surface 106may conduct into the aluminum coating 142. Additionally, the thermalconductivity of silicon is lower than that of aluminum and most othermetals. Hence, a silicon wafer, such as the silicon wafer 136, with ametallic coating, such as the aluminum coating 142, may distribute heatbetter than a silicon wafer with no metallic coating. Thus, the siliconwafer 136 may have an improved heat distribution profile during the etchof the bottom surface 106 according to Step 5 and thus result inpotentially fewer defects in the manifold substrate 102.

At block 330 and as shown at Step 6, the silicon wafer 136 may beremoved from the carrier wafer. Any mounting oil or other substance usedto mount the silicon wafer 136 to the carrier wafer may also be removedfrom the silicon wafer 136. The aluminum coating 142 may also beremoved. In some embodiments, the aluminum coating 142 may be removedusing an aluminum etchant or solvent. Subsequently, the silicon-oxidemasking layer 138 may be removed from the silicon wafer 136. In someembodiments, the silicon-oxide masking layer 138 may be removed using asilicon-oxide masking layer solvent (e.g., a hydrogen-fluoridesolution). The result of the process described in Steps 1-6 is amanifold substrate similar to the manifold substrate 102 depicted atStep 6 and in FIGS. 1A and 1B.

Referring now to FIGS. 4A, 4B, 4C, 5, and 6, a process for minimizingthe extent and time that surface-etched features may be improperlyexposed to chemical etchant are described. FIGS. 4A-4C show an exampleembodiment of a multi-substrate layer cooling device 200. Themulti-substrate layer cooling device 200 may be used to cool a powersemiconductor device, such as, for example, a SiC or GaN powersemiconductor device and may include a cooling fluid flow path 202 thatflows through the multi-substrate layer cooling device 200 to removeheat from the power semiconductor device.

The multi-substrate layer cooling device 200 may include a firstsubstrate layer 204, a second substrate layer 206, and a third substratelayer 208. The first substrate layer 204 may include a top surface 210and a bottom surface 212. The power semiconductor device (not shown) maythermally couple to the top surface 210 of the first substrate layer 204at a cooling location 214. In some embodiments, the cooling location 214may be metallized. Additionally, a cooling array 216 (shown in thedetailed view of FIG. 4C) may be disposed on the bottom surface 212 ofthe first substrate layer 204. The cooling array 216 may face a nozzlearray 220 located on the second substrate layer 206. The coolinglocation 214 may be generally aligned with the cooling array 216 ontheir respective surfaces 210, 212 of the first substrate layer 204 suchthat cooling fluid passing through one or more channels 218 etched intothe cooling array 216 removes heat from a power semiconductor devicethermally coupled to the multi-substrate layer cooling device 200 at thecooling location 214.

Cooling fluid may be impinged on the cooling array 216 from the nozzlearray 220 etched in the second substrate layer 206. The nozzle array 220may include one or more nozzle blocks 222 having one or more nozzlethrough-holes 224 that pass through the thickness of the secondsubstrate layer 206, as particularly shown in FIG. 4B. Still referringto FIGS. 4A-4C, the nozzle array 220 may be disposed within an outletplenum 226 that is etched into a top surface 228 of the second substratelayer 206. The second substrate layer 206 may also include a bottomsurface 230 and a second substrate layer cooling fluid outlet 232 thatextends through the second substrate layer from the top surface 228 tothe bottom surface 230 thereof. Together, the outlet plenum 226 and thesecond substrate layer cooling fluid outlet 232 may form athrough-substrate feature that passes through the entire thickness ofthe second substrate layer 206. The nozzle through-holes 224 of thenozzle array 220, the outlet plenum 226, and second substrate layercooling fluid outlet 232 together form through-substrate features thatextend through the entire thickness of the second substrate layer 206.

The third substrate layer 208 may include a top surface 234 having aninlet plenum 236 formed therein and a bottom surface 238. The thirdsubstrate layer 208 may also include a cooling fluid inlet 240.Together, the inlet plenum 236 and the cooling fluid inlet 240 may forma through-substrate feature that passes through an entire thickness ofthe third substrate layer 208. The third substrate layer 208 may alsoinclude a third substrate layer cooling fluid outlet 242 that may passthrough the entire thickness of the third substrate layer 208 and besubstantially aligned with the second substrate layer cooling fluidoutlet 232.

In some embodiments, the first substrate layer 204, second substratelayer 206, and the third substrate layer 208 may be bonded together. Forexample, the first substrate layer 204, second substrate layer 206, andthe third substrate layer 208 may be directly bonded such that the topsurface 234 of the third substrate layer 208 is bonded to the bottomsurface 230 of the second substrate layer 206 and the top surface 228 ofthe second substrate layer 206 is bonded to the bottom surface 212 ofthe first substrate layer 204.

The particular configuration depicted in FIGS. 4A-4C defines aparticular fluid path for cooling fluid entering the multi substratelater cooling device 200. More specifically, cooling fluid may flowalong the cooling fluid flow path 202. Cooling fluid may flow from anexternal system through the cooling fluid inlet 240 to the inlet plenum236. The bottom surface 230 of the second substrate layer 206 may sealthe inlet plenum 236, thereby preventing cooling fluid from escaping theinlet plenum 236. Said another way, the physical boundary that keepscooling fluid from flowing out of the top of the inlet plenum 236 may bethe bottom surface 230 of the second substrate layer 206. Cooling fluidmay then pass through the nozzle array 220, where it is impinged on thecooling array 216. Cooling fluid may drain from the cooling array 216 tothe outlet plenum 226, where it may be collected before it travels outof the multi-substrate layer cooling device 200 through the secondsubstrate layer cooling fluid outlet 232 and the third substrate layercooling fluid outlet 242.

Some of the herein-listed features of the example embodiment of themulti-substrate layer cooling device 200 may pass through an entiresubstrate layer. For example, as shown in FIGS. 5 and 6, the secondsubstrate layer 206 may include the nozzle through-holes 224 that passthrough an entire thickness of the second substrate layer 206. Thesecond substrate layer cooling fluid outlet 232 and the outlet plenum226 may also form a through-substrate feature through the entirethickness of the second substrate layer 206. As another non-limitingexample, as shown in FIG. 4A, the cooling fluid inlet 240 and the inletplenum 236 may form a through-substrate feature through the entirethickness of the third substrate layer 208 and the third substrate layercooling fluid outlet 242 may be a through-hole through the entirethickness of the third substrate layer 208.

A complication associated with etching multiple through-wafer featuresfrom both sides of a wafer is that not all of the through-wafer featureswill be completed exactly at the same time. That is, the chemicaletchant will etch completely through one or some of the features or aportion of one or some of the features before all of the features arecompletely etched through. For example, in the nozzle array 220discussed with respect to FIGS. 4A and 4B herein, one or more of theindividual nozzle through-holes 224 may be completed before theremainder of the nozzle through-holes 224. The time that the etchantetches through a portion of the through-wafer features may be referredto as the initial through-etch time. Depending on the composition of theetchant, in particular if a gas etchant is used, the etchant may diffusethrough completed through-wafer features into features already etchedinto the opposite side of the silicon wafer after the initialthrough-etch. The already-etched portions may be exposed to chemicaletchant from the initial through-etch time until an etch completion time(i.e., the time at which the etch is completed in substantially all ofthe features). Using the structure described herein as an example, onceetching of one of the nozzle through-holes 224 is complete, gas etchantmay diffuse through the completed one of the nozzle through-holes 224,exposing the outlet plenum 226 and the other features of the secondsubstrate layer 206 to chemical etchant. However, in the particularexample embodiment shown in FIGS. 5 and 6, this outcome may be avoidedby etching the features in the bottom surface 230 of the secondsubstrate layer 206 first.

Accordingly, FIG. 7 shows an example process for etching the featuresshown in the second substrate layer 206 of FIGS. 5 and 6 and FIG. 8 is aflow diagram depicting the example process. It should be understood thatthe particular example embodiment shown in FIGS. 5 and 6 and the exampleprocess shown in FIGS. 7-8 are merely examples and the principlesdisclosed herein are applicable to other etching processes. Brieflyreferring to Step 6 of FIG. 7, the second substrate layer 206 isschematically shown with nozzle blocks 222, nozzle through-holes 224,the outlet plenum 226, and the second substrate layer cooling fluidoutlet 232 removed from the silicon wafer 244 that comprises the secondsubstrate layer 206.

Referring now to FIGS. 7 and 8, as shown at Step 1 and described atblock 805, the silicon wafer 244 may be initially coated with asilicon-oxide mask layer 246. As shown at step 2 and described at block815, portions of the silicon-oxide mask layer 246 may be removed toexpose a pattern of the silicon wafer 244 that will become the featuresof the second substrate layer 206. As shown at step 3 and described atblock 825, the nozzle blocks 222 and the outlet plenum 226 may be etchedfrom the top surface 228 of the silicon wafer 244. The nozzle blocks 222and the outlet plenum 226 may be etched to a nozzle target depth 229 anda plenum target depth 231, respectively. The nozzle target depth 229 andthe plenum target depth 231 may be a fraction of the total thickness ofthe silicon wafer 244 to which the top surface etch of the nozzle blocks222, nozzle through-holes 224, and outlet plenum 226 may extend. In someembodiments, the nozzle target depth 229 and the plenum target depth 231may be the same fraction of thickness of the silicon wafer 244.

As shown at step 4 and described at block 835, the features etched fromthe top surface 228 of the second substrate layer 206 are coated with ametallic coating (e.g., an aluminum coating 248). Once the top surfacefeatures have been etched and coated with the aluminum coating 248, thebottom surface features may be etched at block 845. As shown at step 5,the one or more nozzle through-holes 224 are etched from the bottomsurface 230. As shown at step 6 and described at block 855, thesilicon-oxide mask layer 244 is removed to complete the formation of thesecond substrate layer 206.

It should be understood that the process depicted in FIG. 7 is only oneexample of a process for etching a silicon wafer. The steps describedmay be performed in a different order or in a different manner than thespecific example embodiment described. For example, the nozzlethrough-holes 224 may be etched partway through the second substratelayer 206 by exposing the bottom surface 230 to an etchant and thenetched fully through the silicon wafer 244 by exposing the top surface228 to an etchant. This order is shown in the process depicted in FIG.9. More specifically, a bottom portion 227 of the nozzle through-holes224 may be etched into the bottom surface 230 of the second substratelayer 206 before a top portion 225 of the nozzle through-holes 224 maybe etched into the top surface 228 of the second substrate layer 206, asshown by blocks 925-945 of FIG. 9. The one or more top portions 225 andthe one or more bottom portions 227 of the nozzle through-holes 224 maybe etched through the second substrate layer 206 such that they meet atthe nozzle target depth 229 or at a different depth within the thicknessof the silicon wafer 244.

Similarly, the outlet plenum 226 and the second substrate layer coolingfluid outlet 232 may be etched from both sides of the silicon wafer 244to form a through-wafer feature. More specifically, the second substratelayer cooling fluid outlet 232 may be etched into the bottom surface 230of the second substrate layer 206 to the plenum target depth 231 beforethe outlet plenum 226 may be etched into the top surface 228 of thesecond substrate layer to a plenum target depth 231. In this way, theoverall aspect ratio of each of the features, the etching time, and thedifficulty of etching each of the features may be decreased.

Moreover, as depicted at block 935, a metal coating, such as thealuminum coating 248, may be coated on the bottom surface 230 of thesecond substrate layer 206 before the top surface features are etched.For example, once a bottom portion 229 of the nozzle through-holes 224is etched, the bottom portion 229 of the nozzle through-holes 224 may becoated with an aluminum coating, such as aluminum coating 248, and thenthe top portions 227 of the nozzle through-holes 224 may be etched.Similarly, once the second substrate layer cooling fluid outlet 232 hasbeen etched through the bottom surface 230 of the second substrate layer206, it may be coated with an aluminum coating, such as aluminum coating248 before the outlet plenum 226 is etched. As described herein, thismay help prevent chemical etchant from diffusing throughout the featuresetched into the bottom surface as well as help distribute heat withinthe silicon wafer 244 as it is being etched. However, because the aspectratios of the one or more features on the bottom surface 230 of thesecond substrate layer 206 may be relatively low (for example, whencompared to the features on the top surface 228), it may be unnecessaryto coat the various features on the bottom surface 230 of the secondsubstrate layer 206 with a coating, such as aluminum coating 248, beforeetching the one or more features on the top surface 228 of the secondsubstrate layer 206.

Moreover, by etching the bottom surface 230 of the second substratelayer 206 first, for example by using the example process described inFIG. 9, and then mounting the bottom surface 230 of the second substratelayer 206 to a carrier wafer, and then etching the top surface 228, anyetchant that passes through nozzle through-holes 224 that are completedbefore the others will only react with the aluminum coating 248 in thebottom portion 227 of the nozzle through-hole 224 or be stopped by thealuminum coating 248 from diffusing into the bottom portion 227 of thenozzle through-hole 224 in the first place.

If the features of the second substrate layer 206 are etched from thetop surface 228 first, the outlet plenum 226 and the top half of thenozzle through-holes 224 would already be etched when the bottom surface230 is etched. As the etchant etches through the bottom surface 230 andbegins to complete the second substrate layer cooling fluid outlet 232and the nozzle through-holes 224, the etchant, especially etchant in gasform, would pass through the through-holes that are beginning to form(i.e., forming complete through-holes through the substrate layer) andthe exposed surfaces of the outlet plenum 226 and the nozzle blocks 222would react with the chemical etchant, increasing the exposure of thealready-etched features. This increased exposure may degrade the qualityof the etched features and functionality of the device. Moreover,because the second substrate layer 206 is mounted to the carriersubstrate with mounting oil, this may expose large amounts of mountingoil to chemical etchant, generating a waste product that may bind to thesubstrate and could ultimately affect device performance. In order toavoid these problems, the features etched into the second substratelayer 206 may be etched first from the bottom surface 230.

It should now be understood that embodiments described herein includemethods for etching features or one or more portions of features into asilicon substrate from multiple sides of the silicon substrate andcoating surfaces of the silicon substrate with a passivation layer afteretching through one surface of the silicon substrate before etchingthrough an opposite-side surface of the wafer. The multi-sided etchingof the silicon substrate and the introduction of a passivation layer mayimprove dimensional accuracy of the etch and increase the functionalityof the features formed by the etch.

As used herein, the term “fluidly coupled” refers to two or morecomponents that are in fluid communication, such that a fluid (generallyreferred to within the same paragraph or the context of the descriptionof the component that is fluidly coupled) can pass between the two ormore components. As used herein, the term “thermally coupled” refers totwo or more components in thermal communication such that heat istransferable from the hotter component to the colder of the one or morecomponents by one or more thermal transfer means (e.g., thermalconductivity, thermal radiation, or thermal convection).

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue.

While particular embodiments have been illustrated and described herein,it should be understood that various other changes and modifications maybe made without departing from the spirit and scope of the claimedsubject matter. Moreover, although various aspects of the claimedsubject matter have been described herein, such aspects need not beutilized in combination. It is therefore intended that the appendedclaims cover all such changes and modifications that are within thescope of the claimed subject matter.

What is claimed is: 1.-20. (canceled)
 21. A method of etching featuresin a silicon wafer, the method comprising: placing a patterned mask on atop surface and a bottom surface of the silicon wafer, the patternedmask having a lower etch rate than an etch rate of the silicon wafer;etching one or more top surface features into the top surface of thesilicon wafer through the patterned mask to a target depth plane locatedbetween the top surface and the bottom surface of the silicon wafer at adepth from the top surface; coating the top surface and the one or moretop surface features with a metallic coating; and etching one or morebottom surface features into the bottom surface of the silicon waferthrough the patterned mask to the target depth plane.
 22. The method ofclaim 21, further comprising mounting the silicon wafer to a carriersubstrate before etching the one or more bottom surface features. 23.The method of claim 21, wherein the one or more top surface features areselected from one or more inlet manifolds, one or more auxiliarychannels, one or more cooling fluid inlet channels, and one or more topportions of one or more cooling fluid outlet channels.
 24. The method ofclaim 23, wherein: the patterned mask comprises one or more top surfacefeatures that are removed from a patterned mask top surface located onthe top surface of the silicon wafer, and the one or more top surfacefeatures correspond to the one or more inlet manifolds and the one ormore top portions of the one or more cooling fluid outlet channels. 25.The method of claim 21, wherein the one or more bottom surface featuresare selected from one or more inlet holes and one or more bottomportions of one or more cooling fluid outlet channels.
 26. The method ofclaim 25, wherein: the patterned mask comprises one or more bottomsurface features that are removed from a patterned mask bottom surfacelocated on the bottom surface of the silicon wafer, and the one or morebottom surface features correspond to the one or more inlet holes andthe one or more bottom portions of the one or more cooling fluid outletchannels.
 27. The method of claim 21, wherein the one or more topsurface features and the one or more bottom surface features are etchedusing anisotropic deep silicon etching.
 28. The method of claim 21,wherein the metallic coating is aluminum.
 29. The method of claim 28,wherein coating the top surface and the one or more top surface featurescomprises coating via an aluminum sputtering process.
 30. A method ofetching one or more features in a silicon wafer, the method comprising:placing a patterned mask on a top surface and a bottom surface of thesilicon wafer, the patterned mask having a lower etch rate than an etchrate of the silicon wafer; etching one or more bottom surface featuresinto the bottom surface of the silicon wafer through the patterned maskto a target depth plane located between the top surface and the bottomsurface of the silicon wafer at a target depth from the top surface;coating the bottom surface and the one or more bottom surface featuresetched into the bottom surface with a metallic coating; and etching oneor more top surface features into the top surface of the silicon waferthrough the patterned mask to the target depth plane.
 31. The method ofclaim 30, further comprising mounting the silicon wafer to a carriersubstrate before etching the one or more features into the top surfaceof the silicon wafer.
 32. The method of claim 30, wherein the metalliccoating is an aluminum coating.
 33. The method of claim 30, whereincoating the bottom surface and the one or more bottom surface featurescomprises coating via an aluminum sputtering process.
 34. The method ofclaim 30, wherein the one or more features are etched using ananisotropic silicon etching technique.
 35. A method of etching one ormore through-features into a silicon wafer comprising a top surface anda bottom surface and coated with a patterned mask, wherein the one ormore through-features comprise one or more nozzle through-holes, anoutlet plenum, and a cooling fluid outlet, the method comprising:etching a bottom portion of the one or more nozzle through-holes fromthe bottom surface to a nozzle target depth through the patterned mask;etching the cooling fluid outlet from the bottom surface to a plenumtarget depth through the patterned mask; coating the bottom surface andthe bottom portion of the one or more nozzle through-holes and thecooling fluid outlet with a metallic coating; etching a top portion ofthe one or more nozzle through-holes from the top surface to the nozzletarget depth; and etching the outlet plenum from the top surface to theplenum target depth, wherein: the first of the one or morethrough-features to etch through the silicon wafer is completed at aninitial through-etch time, and the last of the one or morethrough-features to etch through the silicon wafer is completed at anetch completion time.
 36. The method of claim 35, wherein the patternedmask comprises silicon oxide.
 37. The method of claim 35, wherein beforethe top portion of the one or more nozzle through-holes and the outletplenum are etched into the top surface of the silicon wafer, the bottomportion of the one or more nozzle through-holes and the cooling fluidoutlet are filled with mounting oil.
 38. The method of claim 35, whereinthe metallic coating comprises an aluminum coating.
 39. The method ofclaim 38, wherein coating the bottom comprises coating via an aluminumsputtering process.
 40. The method of claim 35, wherein the nozzletarget depth and the plenum target depth are substantially equivalentdepths through a thickness of the silicon wafer.